This request is for continuation of support for our research on mechanisms of input regulation - habituation, perception and stimulus intensity control (reducing/augmenting, strength of nervous system). We have developed measures of individual differences in these capacities for inhibitory control of stimulus input and have completed preliminary studies of their relationship to well-established basic traits of personality or temperament, including neuroticism, introversion and anxiety-proneness. During the present grant year improvements in the measures indicated by completed research will be implemented and some of the more provocative of our prelimanry findings will be replicated and extended. For example, our evidence suggests that "reducing" is an adaptive capacity that can be utilized by stable, low-anxious extroverts when "reducing" is an appropriate (e.g., when required merely to submit to repeated tones or flashes of varying intensity) although these same subjects may "augment" when that set is more adaptive (e.g., when required to give a reaction time response to each tone or flash). We shall therefore explore a paradigm in which both reducing and augmenting, alternately, are adaptive. Beginning during this year, the improved battery of psychophysiological tests will be administered to a large sample of young-adult, same-sex twins and siblings who are participants in the Minnesota Twin Study and many of whom have already been diagnosed for zygosity and have completed Tellegen's Differential Personality Questionaire (11 orthogonal personality/temperament dimensions) and the Activity Preference Questionnaire (a measure of anxiety-proneness, timidity in situations of social stress or physical hazard, which is largely independent of neuroticism, the principal constituent of scales said to measure "trait-anxiety").